Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 180234

Area Forecast Discussion
National Weather Service Sioux Falls SD
934 PM CDT Tue Apr 17 2018

Issued at 918 PM CDT Tue Apr 17 2018

HRRR. RAP, NAMnest, and 00z NAM continue to trend further south
with the approaching shortwave trough. This is supported by the
present location of the trough seen on water vapor. Strong jet
streak and PV anomaly is digging across southwest Nebraska near
the Kansas border, on an eastward track this evening. With the
stronger forcing likewise tracking further south, remaining near
or south of the mid- Missouri valley, have adjusted precipitation
amounts lower in the highway 14 corridor of east central, while
the lower Brule area of south central SD have trended slightly
higher. While this has not altered expected snowfall amounts
significantly, it does bring Gregory county into the 2 to 4 inch
range, so have added them to the advisory. Beadle, Kingsbury,
Sanborn and Jerauld may struggle to reach 2 inches, but given that
they are already in the advisory, did not want to remove them at
this point.


.SHORT TERM...(This evening through Wednesday night)
Issued at 349 PM CDT Tue Apr 17 2018

Current surface analysis shows weak high pressure situated over
Minnesota. Aloft, an upper trough is moving rapidly eastward into
western plains. Another round of snowfall is taking aim across the
area. The upper trough will move into central Nebraska overnight,
before closing off and heading into Iowa on Wednesday. Strong jet
streak of over 100 knots is noted tonight rounding the base of the
upper low near the Nebraska/Kansas border, with a subtle streak
extending northward up the MO River valley of NE/IA. Lift becomes
quite strong with low level moisture increasing rapidly from west to
east tonight. That said, it will take a while for southwest MN to
saturate up so slowed down measurable pops in that area. Between 06Z
and 12Z, very high pops continue to be warranted as they progress
eastward with the deep layer saturation. There is dendricity in the
3-5km layer, and lift is quite strong in this layer between -10 and
-20C. Isentropic lift is strong in the 285-295K layer. Furthermore,
some instability exists in the mid layers late tonight and very
early Wednesday in the Sioux City and Storm Lake areas evidenced by
elevated cape values.

For the details, the models are fairly close with the timing of the
dynamics, moisture, and overall QPF values. The mid level QG forcing
moves squarely over the CWA. There may be a little wintry mix for
locations in extreme northeast NE, extending eastward to Sioux City
and Storm Lake initially overnight tonight before the mid layers
saturate up. But overall the models are under doing the ice
probability function and not introducing snow fast enough as the
profile becomes so deeply saturated. Worried about a bit of a
convective snow burst especially around Storm Lake. Will have to
watch this location for any sort of headline upgrades, because if
they do go convective late tonight or early Wednesday, snow rates
could be an inch an hour for a few hours. This will push Storm
Lake`s snow amount over 6 inches. Conversely due to the track of the
system and the main frontal forcing over our southern zones, our
northern and western zones may receive less snowfall then originally
forecast at about 2 inches, perhaps a bit more. Overall elsewhere, 3
to 5 inches of snow looks like a pretty good bet blending the QPF
off of the various cams, and lowering the snow ratio from what
superblend was giving us. Unlike last weekend, fortunately this will
not be an overly wind driven event. But did increase wind speeds
generally from about I 29 and eastward due to the amount of wind at
925mb and gradient expected. So therefore preferred winds in the
breezy category. Most of the visibility reduction in this event will
be due to falling snow and not blowing snow.

Snow streaks are possible Wednesday afternoon west of I 29 due to
cold air aloft and a cyclonic flow. Further east, high pops will
diminish in the evening east of I 29 as the system moves out of the
area. Concerning temperatures, needless to say they will continue to
be immensely below normal for the daytime high on Wednesday, with
readings only in the 30s to around 40 degrees.

.LONG TERM...(Thursday through Tuesday)
Issued at 349 PM CDT Tue Apr 17 2018

On Thursday and Friday, another upper level low will move into the
southwest United States, with our area dominated by an upper ridge
of high pressure. Sprawling surface high pressure will nudge ever so
slowly eastward, setting up shop over the MS valley and Great Lakes
area on Friday and Saturday. This is important as this high will
usher in a low level fetch of dry air from the east, limiting our
pops as the upper low moves closer on Friday and Friday night. Then
on the weekend, the closed upper low tracks too far south to impact
our weather this weekend. Further out on Monday night and Tuesday,
small pops are warranted with a frontal passage coming down from the

In terms of temperatures, Thursday and Friday will continue to be
well above normal, followed by temperatures closer to normal this
weekend. Warmed up highs on Sunday west of I 29 given the 925-900mb
temperature structure over superblend values. Early next week,
temperatures will be a bit cooler on Tuesday when compared to
Monday as the next frontal passage dips southward.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 636 PM CDT Tue Apr 17 2018

Ceilings will begin lowering late this evening, with light rain
or a wintry mix arriving in south central and Missouri Valley
around midnight. The rain or wintry mix will quickly change to
entirely snow in the early morning hours as it spreads eastward.
Expect widespread MVFR and IFR conditions, with brief LIFR in the
heavier snow bands. Winds will not be extreme, but should average
around 10 to 25 KT during snowfall. Visibility will begin
improving after 15z, but low stratus persists through the TAF


SD...Winter Weather Advisory from 1 AM to 4 PM CDT Wednesday for

     Winter Weather Advisory from 1 AM to 1 PM CDT Wednesday for

     Winter Weather Advisory from 4 AM to 7 PM CDT Wednesday for

MN...Winter Weather Advisory from 4 AM to 7 PM CDT Wednesday for

IA...Winter Weather Advisory from 1 AM to 4 PM CDT Wednesday for

     Winter Weather Advisory from 4 AM to 7 PM CDT Wednesday for

NE...Winter Weather Advisory from 1 AM to 4 PM CDT Wednesday for

     Winter Weather Advisory from 1 AM to 1 PM CDT Wednesday for



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